The present invention falls into the field of reverberation annexation devices which use coiled springs as the raw materials for a signal retardation device.
Generally in a reverberation annexation device wherein a coiled spring is used as raw material for a signal retardation device and spanned between a drive-transducer and a pickup-transducer, the mechanical vibration impressed on one end of the coiled spring by the drive-transducer is helically transmitted along the coiled spring, that is, along the helical direction to the pickup-transducer.
The vibration is transmitter to one end of the coiled spring and picked up by the pickup-transducer, and at the same time it is reflected at the end and transmitted from the reflecting point in the reverse direction, that is, to the drive-transducer. Intermittently repeated reflection of the vibration at both ends of the coiled spring permits the pickup-transducer to pick up a group of signals which are subjected to continuous retardation in time, thereby reverberation sound is obtained.
Although the mechanical vibration is not impressed in the longitudinal direction but in the helical direction of the coiled spring as above described, repeated reflection of the vibration at both ends induces the compressive movement in the longitudinal direction of the coiled spring, which movement is inherent generally in coiled springs. In other words, mode change is effected from the vibration transmission along the helical direction to the compressive movement of the coiled spring. The compressive movement is reflected at both ends of the coiled spring independently from the reverberation signal transmitted along the helical direction of the coiled spring. The repeated reflection of the compressive movement causes the stationary wave to the coiled spring.
Generation of the stationary wave modulates said reverberant vibration. The modulation causes flutter-like sound, which is neither musical nor realistic.
The inventor of the present application has provided a method for eliminating such disadvantages of the system described in U.S. Pat. No. 4,112,396.
In said patent, a part of the coiled spring is constituted as a protrusion which extends further than the coil radius with respect to the center axis of the coiled spring. The protrusion absorbs the compressive movement of the coiled spring, as a result the flutter modulation can be prevented. However, since the protrusion is constituted by utilizing a part of the coiled spring and integral with the coil portion, the protrusion has some of the properties of a coiled spring, that is, it transmits some of the compressive movement induced in the coiled spring. Therefore a number of protrusions must be arranged on the coiled spring in order to completely absorb the induced compressive movement by means of the protrusions. Further the bending work of the coil to produce each protrusion is inevitably carried out by means of handwork, because the protrusion is formed by extending a part of the coil and integral therewith as above described. Therefore it is difficult to carry out the mass production with the same form and quality at low cost.